The invention relates generally to radio frequency (RF) transceivers and more particularly to microwave transceivers with superheterodyne receive frequency conversion stages.
Features of the present invention address problems that may be found, for example, in mobile communications systems, particularly in mobile communication systems which receive data communications via earth orbiting satellite relay stations. Mobile communication systems have to comply in their transmissions to strict regulatory requirements with respect to frequencies and channel separation. In the case of satellite systems, governmental regulations are furthermore based on international agreements which seek to assure the availability of certain channels to designated, licensed uses and users. Strict frequency control in transmission circuits, and a high degree of selectivity and of spurious signal and image signal rejection in reception circuits would be strived for in the design of communication systems.
In conflict to the development of precision communication systems is a practical requirement for cost reduction in order to offer an economically feasible product. The working environments of mobile communications systems are typically those in which the communications systems support and improve already existing prime service functions. The usefulness of an added communications system is, therefore, measured in terms of direct improvement in the productivity of existing prime service functions. However, the cost of the added communications system, such as a mobile system, is factored into an overall operating cost of the existing prime service system when a net increase of productivity which may be attributable to the communications system is determined. As a general proposition, cost avoidance which may jeopardize precise operating characteristics is untenable in a communications environment in which adherence to strict standards is paramount. Therefore, cost containment without relinquishing tight frequency and signal control is a continuous source of motivation for those working in the field of radio transmitted data communications.
Superheterodyne receiver design architectures used in transceiver or transponder circuits are known to share frequency standards with the counterpart radio transmission circuits. However, further improvements in cost and function are desirable in providing a superheterodyne transceiver which may be used in mobile communications systems including mobile communications system with satellite communications relay stations.